Hi Emma,
To confirm that you have the right person you need to work the other way around - from your grandmother backwards, generation by generation, confirming each step from birth/marriage/death certs, wills, census records, etc.
If you have done this to arrive at Gilbert and his wife Maryad or Marid, and not just looked at someone else's tree which might or might not be correct, then yes, Maryad could be written as Marriot(t). You will know that it is the right person because you will have already researched the previous generations and confirmed the names of her children, their married names (if given), and grandchildren (if given) - as they were in this will. There is also a 1717 will for Gilbert KENNAGHT in Malew which you should look at to see whether the children are the same, his wife named, and to pick up further information. It is summarised on Manx Notebook.
However there is no point in looking at these wills unless you have first taken the time to confirm every generation back from your grandmother.
Sue